Known is a method of electrolytic plating the surface of a material, the method including immersion of the treated material, which serves as a first electrode, and of the second electrode in an electrolyte, application of a voltage between them until a plurality of microplasma discharges appears, the micorplasma discharges being evenly spaced across the surface of the treated material, and maintenance of the voltage until a coating of desired thickness is formed. The voltage is increased up to 400 V for basic valve metals and up to 600 V for induced valve metals, the temperature of the electrolyte is maintained in a range of 45-60.degree. C., current density is 250-500 Ma/dm.sup.2 [1].
However, this method has some substantial disadvantages:
low current density entails difficulties in ignition and maintenance of a stable microplasma discharge on the surface of the treated material, in particular for induced valve metals and their alloys, this lowers the quality of the process; PA1 it is not possible to process intricate workpieces or workpieces having a large surface in the suggested electrical regimes; PA1 it is not possible to process workpieces made from carbonic materials (graphite or composites made from it). PA1 S--total surface of the workpiece, dm.sup.2 ; PA1 T--immersion time, min; PA1 i--initial density of the anode current, A/dm.sup.2. PA1 the great thickness of the peripheral technological layer having a relatively porous structure of silicon oxide and aluminium oxide makes it hard to remove it; PA1 the dependence of the immersion speed of the workpiece on the value of the initial density of the anode current applied works only effectively, if the power values (N) of the power sources used are very low (because N=I.multidot.U). In this case only workpieces with limited surface can be coated, so that the preliminary immersion by 5-10% still ensures the ignition and stable burning of microplasma discharges. Due to this fact the possibility of coating large workpieces is limited. PA1 difficulties arise when igniting and maintaining stable microplasma discharges at the same time on large surfaces of a bulky workpiece to be processed or on the surfaces of many small workpieces. Due to this fact the coatings generated do not have uniform thickness and characteristics across the total surface of the workpiece to be processed; PA1 it is necessary to provide a current source of big power in order to maintain a stable discharge on large surfaces of a bulky workpiece to be treated or on the surfaces of many small workpieces, this entails increased energy expenditure during the process; PA1 it is not possible to generate coatings of uniform thickness and characteristics across the whole surface of a workpiece having holes or voids or notches with relation of diameter to length being less than 0.3; PA1 it is not possible to use the method for other non-metallic materials, e.g. graphite or composites made from it. PA1 generating a high-quality coating on large surfaces of one bulky workpiece to be treated or on the surfaces of many small workpieces by simplifying the process of ignition of microplasma discharges and maintenance of their stable burning on the surface to be treated during the whole process while using current sources of moderate power; PA1 obtaining heat-resistant, corrosion-resistant, and wear-resistant dielectric coatings of uniform thickness and characteristics across the total surface to be treated of workpieces and of workpieces with intricate shape, including inner surfaces of holes: PA1 forming a uniform protective coating with a thickness up to 700 .mu.m on workpieces made from aluminium and its alloys with alloying additions or other valve metals, like zirconium, titan, hafnium and their alloys, but also on those materials like graphite and composites thereof. The coating formed comprises the above mentioned properties. PA1 immersion of the surface of the electroconductive material being the anode in the electrolyte fluid or establishing a contact of said first electrode with the electrolyte; PA1 positioning of the second electrode by either immersing it in the electrolytic bath or by using the wall of the bath's body made of electroconductive material as a counterelectrode; PA1 applying an electric regime in the circuit (anode--electrolyte--counterelectrode), including the application of an initial amperage of the polarizing current, maintenance until formation of a coating of required thickness on the surface of the workpiece to be treated, switching off the forming voltage; PA1 taking out the workpiece; PA1 S.sub.H --is the portion of the workpiece that is immersed in the electrolyte, dm.sup.2 ; PA1 N--is the output power of the power source, Volt.multidot.Ampere; PA1 A--is an empiric parameter, depending on the composition of the material to treated, the electric regime used, and the composition of the electrolyte, A=550 to 5000 V; PA1 i--is the minimal current density at which microplasma discharges appear and at which the process of microplasma oxidation is stable, A/dm.sup.2. PA1 I--maximal current magnitude (A), which can be provided by the source; PA1 N--output power of the source (V.multidot.A); PA1 U--value of the stabilized voltage, the magnitude of which is higher than the breakthrough voltage V of the dielectric coating applied. PA1 the method of immersing the workpiece in the electrolyte, which is carried out: PA1 the sequence and character of realizing the electric regime of the microplasma processing; PA1 the relation of the cathode and the anode current components of the electric regime, the duration of the anode and cathode impulses or bursts and the duration of the pauses between them.
Known is also a method of electrolytic micro-arc plating of a silicate coating onto a aluminium workpiece [2]. The method comprises steps of forming a coating by preliminary dipping a part into the electrolyte by 5-10% of its surface area at initial current density of anode current, equal to 5-25 A/dm.sup.2 and performing further dipping uniformly with a rate, determined by the relation S/T=0.38+1.93 i,
wherein
This method has some substantial disadvantages:
The most similar method in terms of the main features is a method for forming coatings by electrolyte discharge[3]. This method for forming relatively thick composite coatings on a region of the surface of a metallic workpiece comprises exposing the surface region to an electrolyte fluid, either by immersion or by spraying the electrolyte against the surface region. A preferred electrolyte fluid is an aqueous solution including an electrolytic agent, a passivating agent and a modifying agent in the form of a solute or a powder suspended in the solution. A voltage signal is applied to induce a current flow of constant magnitude between the metallic member and the electrolyte fluid so that the metallic member interacts with the passivating agent to form a passive oxide layer on the surface region. The voltage signal increases in magnitude until local voltage reaches a breakthrough level across separate highly localized discharge channels along the surface region of the metallic member. At this breakthrough level, localized plasmas including components of the oxide layer and the modifying agent form near the discharge channel and react to form the coating. At some point after the discharges appear, the signal is changed to a series of unipolar anodic pules interspersed with the cathodic pulses which serve to stabilize the growth of the coating.
Thus, the known method comprises the steps of establishing a contact of the material serving as first electrode and of the second electrode with the electrolyte; applying a voltage between the electrodes in the regime of ignition of a plurality of microplasma discharges and maintaining the material in the electrolyte at given electrical parameters, thus, generating a coating of desired thickness.
The substantial disadvantages of this methods are: